The invention relates to high-efficiency multilevel diffractive optical elements and particularly to fabrication techniques that produce arbitrary multilevel-phase diffraction elements for reflection mode EUV devices.
Multilevel-phase diffraction elements play a very important role in the realm of the optics. Examples of such devices include diffusers, kinoforms, and phase grating such as sinusoidal and blazed gratings. With the advent of multilayer reflectors, optical systems have been pushing towards ever-shorter wavelengths.
Currently, most EUV diffraction elements are of the binary amplitude type, which severely limits their flexibility. Theoretically, on-axis diffractive phase elements consisting of a grating having a given period can achieve 100 percent diffraction efficiency. To achieve this efficiency, however, a continuous phase profile within any given period is necessary. The theoretical diffraction efficiency of this surface profile is also relatively sensitive to a change in wavelength. The technology for producing high quality, high efficiency, continuous-phase-profile reflection diffractive elements working at EUV wavelengths does not presently exist.
A compromise that results in a relatively high diffraction efficiency and ease of fabrication is a multilevel phase grating. The larger the number of discrete phase levels, the better the approximation of the continuous phase function. The multilevel phase surface profiles of the grating can be fabricated using standard, semiconductor integrated circuit fabrication techniques.
A typical binary optics fabrication process starts with a mathematical phase description of a diffractive phase profile and results in a fabricated multilevel diffractive surface. The next step is to transfer the phase profile information into the substrate. This can be achieved through a variety of methods including conventional and electron-beam lithography methods. Typically this is done by decomposing the desired multilevel pattern into a series of binary patterns and performing multiple lithography steps.
A substrate of the desired material, such as silicon or glass, is coated with a thin layer of photoresist. A first pattern is transferred to the photoresist using a standard lithography technique such as, for example, projection, contact, or electron-beam lithography. The photoresist is developed, washing away the exposed resist and leaving the binary pattern in the remaining photoresist. This photoresist will act as an etch stop.
A reliable and accurate way to etch typical substrate materials is reactive ion etching. The process of reactive ion etching anisotropically etches material at very repeatable rates. The desired etch depth can be obtained very accurately. The anisotropic nature of the process assures a vertical etch, resulting in a true binary surface relief profile. Once the substrate has been reactively ion etched to the desired depth, the remaining photoresist is stripped away, leaving a binary surface relief phase grating.
The process may then be repeated using the next binary pattern. The partially patterned substrate is recoated with photoresist and exposed using the second binary pattern, which has half the period of the first mask. After developing and washing away the exposed photoresist, the substrate is reactively ion etched to a depth half that of the first etch. Removal of the remaining photoresist results in a 4 level approximation to the desired profile. The process may be repeated a third and fourth time with binary patterns having periods of one-quarter and one eighth that of the first mask, and etching the substrates to depths of one-quarter and one-eighth that of the first etch. The successive etches result in elements having 8 and 16 phase levels. More masks than four might be used, however, fabrication errors tend to predominate as more masks are used.
This process produces a multilevel surface relief structure in the substrate. The result is a discrete structure approximating the original idealized diffractive surface. For each additional lithography step used in the fabrication process, the number of discrete phase levels is doubled.
After only four processing iterations, a 16 phase level approximation to the continuous case can be obtained. The process can be carried out in parallel, producing many elements simultaneously, in a cost-effective manner.
A 16 phase level structure can theoretically achieve 99 percent diffraction efficiency. The residual 1 percent of the light is diffracted into higher orders and manifests itself as scatter. In many optical systems, this is a tolerable amount of scatter. The fabrication of the 16 phase level structure is relatively efficient due to the fact that only four processing iterations are required to produce the element.
Binary optical elements have a number of advantages over conventional optics. Because they are computer generated, these elements can perform more generalized wavefront shaping than conventional lenses or mirrors. Elements need only be mathematically defined: no reference surface is necessary. Therefore, wildly, asymmetric binary optics are able to correct aberrations in complex optical systems, and elements can be made wavelength-sensitive for special laser systems.
Recently, extreme ultraviolet (EUV) wavelength systems have attracted significant interest due to their applicability to next-generation projection lithography for semiconductor manufacturing. It would be highly desirable to have multilevel-phase diffraction elements that work at EUV wavelengths. Efficiency concern generally limit these EUV devices to being reflection devices because of the significant attenuation imparted by all materials upon transmission on EUV light through the material. Unfortunately, the method described above is not well suited for the fabrication of these reflection devices at EUV wavelengths. The problem is in the extremely high tolerances required of the individual step heights. For an 8-level near-normal incidence reflective EUV diffraction element the step height control would have to be a small fraction of a nanometer. Such etch control is extremely difficult to achieve in practice. The present invention describes a method well-suited to the fabrication of reflective EUV diffraction elements.
The present invention is directed to techniques for fabricating reflective EUV diffraction elements. This goal is achieved by fabricating a well-controlled, quantized-level, engineered surface, which is subsequently overcoated with a conventional EUV reflection multilayer. For a typical EUV multilayer, if the features on the substrate are larger than 50 nm, the overcoated multilayer will be conformal to the substrate. Thus, the phase imparted to the reflected wavefront will closely match that geometrically set by the surface height profile. This avoids the difficulties involved in trying to directly pattern into the reflective EUV multilayer and allows the deposited multilayer to effectively smooth out high-frequency (undesired) roughness which may be present on the patterned substrate.
Accordingly, one embodiment the invention is directed to a method of fabricating an EUV diffraction element that includes the steps of:
(a) forming an etch stack comprising alternating layers of first and second materials on a substrate surface where the two material can provide relative etch selectivity;
(b) creating a relief profile in the etch stack wherein the relief profile has a defined contour; and
(c) depositing a multilayer reflection film over the relief profile wherein the film has an outer contour that substantially matches that of the relief profile.
In a preferred embodiment, step (b) includes forming a relief profile having at least three levels wherein each level is formed by:
(i) forming a resist film on top of the stack;
(ii) exposing one or more patterned regions and developing to uncover one or more regions of the stack;
(iii) etching one layer of the stack where uncovered of resist; and
(iv) repeating steps (i) through (iii) at least once up to the number of stack layers available.
In another preferred embodiment, the multilayer reflection film comprises alternating layers of a third material having a refractive index and a fourth material having a refractive index that is larger than that of the third material.
In yet another embodiment, the invention is directed to an EUV device including a multilevel diffraction element that includes:
(a) a substrate having a stack comprising alternating layers of first and second materials on a substrate surface wherein the stack defines a relief profile having a desired contour; and
(b) a multilayer reflection film that covers the relief profile wherein the film has a contour that substantially matches that of the relief profile.